1. Field of the Invention
The present invention relates to lead titanate based piezoelectric ceramic materials having superior Curie temperature, hysteresis and pressure-electric charge output characteristics, etc., which are especially useful in the fields of pressure sensors, acceleration sensors and filters, etc.
2. Description of the Related Art
Piezoelectric materials can be used as pressure sensors or acceleration sensors, by making use of the piezoelectric effect thereof. Namely, when a pressure is applied to a piezoelectric material an electric charge is generated in accordance with the pressure (deformation), and it is possible to detect this electric charge and thereby measure the pressure or the acceleration.
Lead titanate piezoelectric materials of a composition approaching that of this present invention are disclosed in Japanese Examined Patent Publication (Kokoku) No. 60-21941, Japanese Unexamined Patent Publication (Kokai) No. 55-151381 and Japanese Unexamined Patent Publication (Kokai) No. 63-151667.
The piezoelectric materials used as pressure sensors and acceleration sensors must be such that the pressure - electric charge output is stable, and especially in environments where changes in temperature occur, the characteristics must be stable within the used temperature range.
To date, however, a piezoelectric material satisfying such conditions in a practical sense has not been obtained.
The lead titanate piezoelectric materials disclosed in the aforementioned Japanese Examined and Unexamined Patent Publications were developed principally for resonators, and consequently, the characteristics required for pressure sensors and acceleration sensors were not evaluated. For example, the piezoelectric material of Japanese Examined Patent Publication (Kokoku) No. 60-21941 has a low Curie temperature and the temperature variation of the piezoelectric characteristics is considerable, and the piezoelectric material of Japanese Unexamined Patent Publication (Kokai) No. 55-151381 is such that the hysteresis of the pressure - electric charge characteristics at high temperatures is considerable. The temperature variation of the resonance frequency is small in Japanese Unexamined Patent Publication (Kokai) No. 63-151667, but the temperature variation factor of the dielectric constant is considerable, and thus ultimately the temperature variation factor of pressure - electric charge output becomes remarkable.
The present invention is intended to provide a satisfactory piezoelectric ceramic material for use principally as a pressure sensor or acceleration sensor, under such circumstances.